1. Field of the Invention
The present invention relates to a vehicle steering control system and a control method therefor. In particular, the present invention relates to a control device that controls the operation of an electric power steering device.
2. Description of the Related Art
In vehicles such as automobiles, an electric power steering device, which drives an electric motor in response to the steering torque applied to the steering wheel by the driver, applies a steering assist force to a steering mechanism including the front wheels. Such an electric power steering device has a speed reducer for reducing the rotational speed of the electric motor for outputting the steering assist force, and a gear mechanism, such as a rack-and-pinion mechanism. A grease is applied to the gear mechanism to improve the movability of the gear mechanism. It is known that the viscosity of the grease increases upon exposure to a low temperature (in particular, a temperature below freezing). When the viscosity of the grease increases, the preload of the gear mechanism increases and the steering wheel becomes heavy to turn. This is not preferred because the steering feel for the driver is adversely affected.
To solve the problem, a viscosity coefficient related value related to a viscosity coefficient of the steering system is estimated and the steering assist force is increased by correcting it according to an increase in the viscosity coefficient related value and the steering angular speed in the technology described in JP-A-2003-285752. Then, when the viscosity of the grease has increased, the steering assist force can be compensated. JP-A-2004-352090 describes a technology in which a current is passed through a d-axis when the environmental temperature of the gear mechanism is below a prescribed value. The temperature of the electric motor is thereby increased, which decreases the viscosity of the grease.
However, because the viscosity of grease increases depending on temperature, it is considered possible to let the driver have more natural steering feel by correcting the steering assist force using the temperature directly. On the other hand, providing another temperature sensor in the gear mechanism is not necessarily the best option because it requires the cost relating to the space in which the temperature sensor is installed and the cost for changing the configuration of the existing hardware to be connected to the temperature sensor in addition to the cost of the temperature sensor itself.
Also, a rack coaxial type power steering device in which the rotation of an electric motor is converted into reciprocating motion of a rack bar by a ball screw, in particular, tends to generate noise due to vibration of the ball screw. To reduce noise in an ordinary temperature range, the vibration of the ball screw needs to be reduced. In this case, however, the friction increases and results in degradation of steering feel in a low temperature range. That is, when priority is given to the steering feel in a low temperature range by such a method, the problem of noise in an ordinary temperature range occurs.